webserver: Fix garbage output on debug print (etr#244)

The `upload_data` we get in the callback from libmicrohttpd might not be
null terminated.

Fixes: etr#242
Signed-off-by: Alexander Dahl <ada@thorsis.com>

Author: LeSpocky

src/webserver.cpp

@@ -525,7 +525,7 @@ MHD_Result webserver::requests_answer_second_step(MHD_Connection* connection, co
 
     if (mr->has_body) {
 #ifdef DEBUG
-        std::cout << "Writing content: " << upload_data << std::endl;
+        std::cout << "Writing content: " << std::string(upload_data, *upload_data_size) << std::endl;
 #endif  // DEBUG
         mr->dhr->grow_content(upload_data, *upload_data_size);